Vulcanized and crosslinked elastomers and polymers are well known for use as insulation compositions. These compositions are usually cured by the application of heat although other means for curing may be utilized. Steam has been utilized as a heating medium although higher temperatures involve progressively disproportionally greater increases in pressure.
The problems inherent in using steam led to the use of ultra high velocity gases under moderate pressure to vulcanize and/or crosslink elastomers or polymers. The use of a steam of gas at high temperature effectively penetrates through and/or drives away the thermally insulating static boundary layer or stagnant film of air or other gas molecules residing on or about the curable composition. This results in a sweeping of any air or gas that can act as an insulation barrier to the application of heat and thus obstruct efficient heat transfer. The use of high velocity gases for curing elastomers is described in U.S. Pat. No. 4,069,286 which is incorporated by reference.
It has been found that the hot gas curing system has certain limitations when it is necessary to cure at extremely low temperatures, i.e. in the area of 250.degree. F. Low temperature cures are required when external rubber jackets are applied to composite cables that include internal thermoplastic materials that deform and exhibit flow properties under conditions used to cure rubber compounds. To solve this problem, it has been proposed to use lead curing systems and salt bath pressurized cures to achieve low temperature cures.
The applicants have discovered that the addition of a mixture of a dialkylthiourea and an aryl guanadine at the point of extrusion may be used to reduce the curing temperature to about 250.degree. F. and still obtain a proper cure without causing premature set up of the rubber compound.
In the prior art, dialkylthioureas and aryl guanadines have been used separately to accelerate the cure of rubber compositions. These materials have been used as ultra-accelerators for curing systems that have been used for curing latex rubber compositions. These compositions may be cured in short cycles because of the absence of the mill scorch problem that is experienced in dry rubber compounding. Generally ultra accelerators have not been used in connection with the curing of dry compounded materials because of the scorch problem that results when dry compounded formulations containing fast curing or "hot" curing systems are handled using conventional equipment and procedures which require relatively high curing temperatures.
Therefore it is a primary object of the invention to provide a method for the rapid, low temperature curing of a dry compounded rubber composition.
It is also an object of this invention to provide a method of rapidly curing a dry compounded rubber composition using a fast moving gaseous stream.